Solar heating systems generally fall into two broad catagories. They include nonimaging concentrators that do not focus the sun and imaging or focusing concentrators that operate on the same principle as a flashlight and mirror or magnifying glass. The present invention is directed primarily to an improved feature of a solar transducer used primarily in nonimaging concentrators for high temperature applications. Many of these nonimaging concentrators use a vacuum system in which an insulated metal tube, maintained in a tubular glass envelope under vacuum collects heat in a fluid passing through the metal tube. Many early attempts to use this type of metal tube solar energy transducer have failed because of the differential expansion rates between the metal tube and its enclosing glass vacuum bottle. The differential expansion rates of the metal and glass frequently cause the seal between the metal and glass members to fracture. In one system, a metal U-shaped tube was positioned within a transparent glass envelope with the ends of the U-shaped metal extending from the glass envelope. This system failed for several reasons, including differential expansion referred to above and because of structural loading of the U-tube under heat-stressed conditions. There have, however, been some successful vacuum insulated solar energy collectors. These systems are expensive since they normally eliminate all metal-to-glass seals, and rely upon an all-glass system. In addition to the inherent expense involved in such arrangements, such systems are also subject to greater damage due to glass breakage as well as problems for interfacing such systems with conventional metal heating system piping to which most systems have to be connected.
Other systems have been attempted, but none have been particularly satisfactory in providing a low-cost unit which is not likely to fracture when subject to heat stresses.